This is the cheapest deck to farm Pegasus Lvl 40 probably. Although win rate is not high relative to other farming decks with the Unhappy Girl or Dream Clown, it will work for you who did not roll the 1st and the 2nd card boxes and started the game recently.
With the dinosaur kingdom skill, most of your dinosaur monsters will have high enough attack to beat most of Pegasus' monsters with ease, use Otohime for his higher attack monsters. Tribute summon Blue-Eyes at any point in the duel to get the tribute summon bonus. Ritual summon Cú Chulainn the Awakened using BEWD as the sacrifice to make sure he is in the graveyard, activate Cu's ability and equip him with Ritual Weapon and Metalmorph to deal over 5,000 damage. You can also replace ritual weapon with Gift of the Martyr for more flexibility.
Get "Titan Showdown" skill!
Using Rex to farm Pegasus will also help you get Rex's exclusive skill "Titan Showdown". It's a skill that can be essential in some farming decks by doubling damage dealt(or receive), making any monster that can reach 5,000 attack to be able to deal 10,000 for higher score bonus.
To get better score with higher win rate
To increase your score, use glossy or prismatic cards. Every time you use a glossy card, it's worth 100 points, 200 for prismatic cards. Once you get better cards, you can replace cards like de-spell for better ones such as Wild Tornado or use Master of Oz as the fusion monster.
I'd suggest sticking with the De-Spells over Wild Tornado, so you don't have to worry about Mimicat blowing up your Fusion Weapon on the last turn.